The present invention relates to current control methods and circuits for maintaining a desired average current through the windings of an electronically-commutated motor (ECM) by employing ON/OFF duty cycle control switching.
Electronically-commutated motors typically include a permanent magnet motor and a plurality of field windings which are sequentially energized to produce a rotating electromagnetic field. Such motors are advantageously adaptable to precise control of speed and direction. High torque is available at all speeds, and rotor position is often repeatable without error and independent of load.
A variety of techniques have been proposed for controlling such motors, such as the technique disclosed in commonly-assigned Alley et al U.S. Pat. No. 4,250,435, the entire disclosure of which is hereby expressly incorporated by reference. In the exemplary application disclosed in Alley et al U.S. Pat. No. 4,250,435, an electronically-commutated motor is employed in a direct-drive domestic clothes washing machine. This particular application well illustrates the capabilities of electronically-commutated motors. In general, motor speed and direction are controlled by controlling the frequency and order of commutation, i.e., the sequential energization of the various windings in the motor. At the same time, motor winding current is controlled in order to dynamically match motor torque to the load requirements. As is demonstrated in the above-incorporated Alley et al Pat. No. 4,250,435, commutation frequency and motor winding current control are closely related to each other in practical and efficient ECM motor control systems.
Thus, at least one form of ECM control system dynamically establishes or commands a particular motor winding current to be maintained, and ON/OFF duty cycle control circuitry maintains the desired average current by energizing and de-energizing the motor windings through one or more controlled switches, such as switching transistors.
The present invention is directed to techniques for effecting current control, and it is assumed that the desired current to be maintained is established or commanded by other means, such as are disclosed in Alley et al U.S. Pat. No. 4,250,435. Similarly, it is assumed that suitable means are provided for establishing the rate of commutation, and thus velocity of the motor.
One prior art approach to motor winding current control is to measure motor winding current at all times during the ON/OFF switching process. Upper and lower current threshold values are established as a function of the desired average current to be maintained, which typically is midway between the two current thresholds. In operation, the switching transistor is turned ON energizing one or more of the windings. Current then rises during a rise time interval. When the upper current threshold is reached, the switching transistor is turned OFF to de-energize the motor winding or windings. Collapsing magnetic fields then induce a voltage which drives flyback current which flows through one or more free-wheeling diodes. The flyback current decreases during a decay time interval. When this flyback current reaches the lower current threshold, the switching transistor is turned ON to again energize the selected winding or windings. This process repeats indefinitely. Specific examples of this approach are disclosed in, for example, Pritchard U.S. Pat. No. 4,350,943 and Nielsen et al U.S. Pat. No. 4,409,524.
One disadvantage of this approach, depending upon the topology of the particular circuit involved, is that it may be difficult to measure actual motor winding current during the decay interval, i.e., the flyback current. Thus, the Nielsen et al circuit, which has a free-wheeling diode connected generally in parallel with each switching transistor, separates the ground return paths of the switching transistors and the free-wheeling diodes, and provides an individual current-sensing resistor in series with each of these ground return paths. This permits rise current as well as decay current to be directly sensed.
Pritchard illustrates the complexity of the problem. Thus, the Pritchard circuit attempts to control motor current between two thresholds, and employs a compensation technique which employs a specialized trifilar wound transformer.
A variant approach to controlling current in a motor winding is disclosed in Anderson U.S. Pat. No. 4,107,593. The Anderson circuit includes a free-wheeling diode in parallel with each of three stepper motor windings which are energized through individual switching transistors. Anderson employs only a single current sensing circuit to measure current when one of the transistor switches is turned ON to energize a particular winding. As in the upper and lower current threshold approach described above, the transistor switch is turned OFF when current reaches a predetermined threshold. However, rather than measuring winding current during the decay or flyback current interval for comparison against a lower current threshold, a timing circuit maintains the switching transistor OFF for a fixed predetermined time period, after which energization is allowed to continue. While motor winding current is thus effectively limited, this Anderson control approach lacks the necessary flexibility for use in an electronically-commutated motor intended for operation over a wide range of speed and loading conditions. Specifically, the fixed OFF time interval of the Anderson circuit effectively limits the maximum average current which can be supplied to the motor winding. Perhaps more significantly, unless motor operating conditions are known and substantially unvarying, motor winding current cannot be well-controlled to a particular average value since there is no particular lower current threshold. While this may be perfectly acceptable in the particular stepper motor application which Anderson envisions, it is contrary to a motor control system whose object is to maintain optimum motor operation at all times.